I can tell you what I'm in the process of doing with the thought that it may or may not be something you want to consider. We just purchased an '05 SC430 for my wife for her 70th birthday. It has the stock wheels 18"x8". We both want a smoother, less harsh ride so I've decided to replace the stock wheels with some Work VS-KF 17"x17" wheels and higher profile tires. And, since we are moving away from run flat tires we're going to add a donut spare.
First the wheels....We wanted a high quality wheel that was light weight, but durable and one which didn't require my frequent polishing of the aluminum lip. We chose the Work wheels because of their stellar reputation and because there are enough of them out there to be able to pick up a replacement wheel should we ever ding one of our set. I found a set of used wheels 17"x7" with a +35 offset on Ebay. The stock lug nut pattern is 5x114.3 and these match. The stock offset is +45 so these wheels will set the tires outboard about 3/8" further See:
The stock wheels are hub centric, meaning the wheel is mounted and centered on the axle by having a hub extension on the axle which just barely fits the wheel. The wheel is thus centered on the axle and the weight and stress are carried by the hub, while the lug nuts just keep the wheel tight to the mounting surface. The new wheels have a different hub bore than the stock wheel, so I'll be purchasing round hub centric shims to fit the new wheel hubs to the existing axle hubs. This will make sure the wheel and tire is centered on the hub. I figure the new wheels have just the right amount of bling to suit my wife. See this photoshop pic of similar wheels on her car:
I considered going to 16" wheels to allow for a taller sidewall tires, but I couldn't be sure they would clear the brake calipers, so I went with the 17's.
By going away from the run flat tires, I would need to come up with a spare tire. But first, I had that annoying Tire Pressure Monitoring System light on my dash. Evidently one or more of the batteries in the sending units is kaput so I just disabled the TPMS system following the guidelines posted on these forums. Next, I found a spare which I think will fit: see:
Some of the considerations in choosing this wheel is 1. it is 17" in diameter and doesn't have any protrusions which will foul my brake calipers. 2. Looking from the backside of the spare, there appears to be enough "outward dish" in the wheel to clear the brake calipers. 3. Being an alloy wheel, it is light weigh and easy to lift in and out of the trunk. 4. a true donut spare which will easily fit in the trunk and not foul the retracted hardtop. 5. It is from a 2012 car so I won't need to purchase a new "donut" tire to fit it because of age of the tire. I'll follow the helpful guidlines posted in these forums to mount the tire in the trunk. See: http://www.clublexus.com/forums/sc-4...unk-space.html
Now for tires, checking manufacturer specs, our original tires should have a diameter of close to 25.7" so I wanted to keep the new tires close to that diameter to minimize speedometer error. Since I've always trusted Michelin tires I started looking there. Based on Excellent Luxury Quiet
Ride, Very low Rolling Resistance (for good gas mileage), excellent wet traction, and Extremely Durablity, I chose the Michelin Primacy MXV4 in a 215/55 r17. See: http://www.michelinman.com/tire-sele...FQioPAodfHwA1A
They will fit wheel widths from 6"-7.5", The diameter is 26.3" so the speedo error is only about 1 mph at 60 mph, and shopping around (TireRack and Ebay), I should be able to get 4 shipped in for about $680. Evidently, these are not a popular
size tire so they are priced lower than some other tires close to the same size. We'll go with the "H" rated tires. I did mention my wife is turning 70.
This whole change over is still in the process, waiting for the wheels to arrive, but I should substantially improve the ride, noise level, and gas mileage over the run flats.
It isn't like this is a big expenditure. We'll be spending about $1700 for wheels and new tires mounted on the car. The tires that are on the car have about 90% tread so we will put them on Craigslist, and same with the OEM wheels and hubcaps which might put $500 back in the bank.
Update: The Hyundai Genesis spare tire came in today and I did a quick fit and test and it passed with flying colors. There is plenty of room to clear the brake calipers, and it does not require an appropriate sized hub centric ring spacer to center the wheel. The Hyundai wheel comes with tapered lug nut seats and the stock lug nuts have a tapered portion at the end. The tapered seats and tapered nuts center the spare on the hub quite nicelly.